The present invention relates generally to the nondestructive evaluation of engineering materials, and more particularly to the use of ultrasonic synthetic aperture focusing technique for detecting and characterizing flaws in engineering materials having planar and non-planar geometries.
Nondestructive evaluation (NDE) of engineering materials for flaws is an important step in ensuring product quality. A commonly used NDE technique is ultrasonic inspection. Typically, in ultrasonic inspection, an ultrasonic transducer scans the surface of the engineering material, sending either a focused or unfocused ultrasound beam towards the material. The transducer receives an ultrasound beam reflecting from the material. The maximum ultrasound signals are recorded to form a C-scan image and then compared to a threshold. If the maximum ultrasound signals exceed the threshold, then an alarm is activated. C-scan imaging provides simple and high quality images, however there are some limitations. For example, the lateral resolution of C-scan images is inversely proportional to the diameter of the transducer. Since the diameter of the transducer cannot be made arbitrarily large due to the accompanying increase in capacitance, the resolution in C-scan images will be limited. Another limitation is that the depth of field is relatively small for high resolution images, so mechanical scanning of multiple transducers focused at different depths is necessary to inspect thick sections of material.
Many of the limitations of C-scan imaging are circumvented by using a synthetic aperture focusing technique (SAFT). The idea behind SAFT is to synthesize an aperture by coherently combining the data generated at a plurality of scanning positions. In a typical SAFT implementation, a series of A-scans (i.e., RF wave forms representing echoes) are generated from different scanning positions. Subsequently, the A-scans are coherently combined to form focused images of the object's interior. The coherent combination is generally performed either in the spatial-frequency domain using a wave-theoretic approach or in the time-domain using the delay-and-sum approach. The effective aperture of a SAFT reconstruction is determined by the scanning positions that contribute to each reconstruction point. Thus, a wider beam pattern results in a larger effective SAFT aperture. Since SAFT imaging achieves focusing by processing the digitized wave form data, mechanical scanning is not necessary to image several depths as in C-scan imaging. However, a problem with present SAFT implementations is that they are limited to the detection and characterization of flaws in planar or simple geometries. Many NDE problems, especially those found in aerospace involve parts of complex geometry and the conventional SAFT implementations are unable to image these geometries because of transducer imaging limitations.